We plan to continue our studies on acidification on reabsorption of NaCl and water in the proximal convoluted tubule of rat and rabbit kidneys. With respect to the mechanism of acidification we plan to: 1) examine the nature of the ion (H ion, HCO minus over 3, OH minus) across the luminal and basolateral membranes, 2) identify and quantify the pathways for bicarbonate backleak from blood to lumen, 3) define the specific roles of carbonic anhydrase present in the cytoplasm and in luminal and basolateral membranes, 4) develop a kinetic model for acidification, and 5) examine the mechanism by which homeostatic regulatory factors exert their effects. In the second part of the proposal we plan to examine the mechanism by which NaCl and water are reabsorbed, the role of organic solutes (e.g., glucose, amino acids, HCO minus over 3), and anion concentration gradients in this reabsorptive process, and the manner in which certain regulatory factors (e.g., peritubular oncotic pressure, acid-base status, potassium, PTH, etc.) influence the rate of this reabsorptive process.